This invention relates generally to the re-charging of electrochemical cells having lithium-alloy negative electrodes and is particularly directed, but not limited, to protecting a high temperature lithium-alloy/metal sulfide cell and batteries from damage due to overcharging.
Lithium-alloy/metal sulfide cells are characterized by high storage capacity and power capability per unit weight. The individual cells are typically of the high-temperature type and are coupled in series and/or parallel to form batteries for the storage of electric power.
Re-charging of lithium-alloy/metal sulfide cells is typically carried out in a region where the voltage of the negative electrode is maintained at a constant, well defined level. If this voltage level is exceeded, the positive electrode may reach a higher potential condition in which iron chloride is formed, where the positive active material or current collector is an iron-based material, with the iron chloride then dissolving in the electrolyte. The dissolved iron chloride is then transferred via the electrolyte to the negative electrode or to the separator area resulting in destruction of the positive electrode and a shorting condition within the cell when the iron precipitate bridges the two electrodes. These conditions caused by cell overcharging will result in the destruction of the cell. With a plurality of lithium alloy/metal sulfide cells coupled together to form a battery, overcharge of the weak cells of the battery is a serious problem whenever there is a disparity in the utilizable capacities of the cells. Maintaining the charge voltage at or below a permissible voltage limit that does not produce any harmful effects on cell cycle life while allowing for the simultaneous attainment of equal capacity in each serially connected cell of a metal sulfide battery, cell capacity equalization, over many cycles are difficult tasks. Various approaches have been adopted to avoid cell overcharging and prevent cell damage and short circuit.
One prior art approach for battery overcharge protection makes use of Li.sub.2 S in the positive electrode to provide a chemical overcharge tolerance by a polysulfide shuttle mechanism. The polysulfide shuttle mechanism is limited to cells having disulfide, e.g., FeS.sub.2 or NiS.sub.2 positive electrodes and is not applicable to lithium-alloy/monosulfide cells. U.S. Pat. No. 4,324,846 to Kaun et al utilizes a ternary alloy of iron-aluminum-lithium or nickel-aluminum-lithium or cobalt-aluminum-lithium to provide a specific overcharge capacity to afford a limited overcharge protection. Electric overcharge protection has also been employed such as taught in U.S. Pat. Nos. 4,079,303 to Cox and 4,238,721 to DeLuca et al. These patents disclose electrical systems for charging multicell storage batteries in a manner which prevents individual cell overcharging. The former patent removes any cell from the charging cycle which reaches a predetermined charge voltage limit, while the latter equalizes the charge of each individual cell at a selected full charge voltage by shunting current around any cell having a voltage exceeding this selected voltage.
The present invention does not employ extra electrical circuitry to prevent detrimental overcharge of the cell. The present invention is particularly adapted for preventing overcharge of a battery comprised of a plurality of lithium-alloy/metal sulfide cells by electrochemical means.